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Reference: Biol. Bull. 192:243-252. (April, 1997) Morphology and Development of Odostomia columbiana Dall and Bartsch (Pyramidellidae): Implications for the Evolution of Gastropod Development RACHEL COLLIN1 * AND JOHN B. WISE2 1 Department of Zoology, University of Washington, Box 351800, Seattle, Washington 98195; and 2 Houston Museum of Natural Science, 1 Herman Circle Drive, Houston, Texas 77030 Abstract. Although pyramidellid gastropods are a phy- plaining the evolution of gastropod cleavage type and logenetically important group of diverse and abundant larval heterochrony. Unequal cleavage and larvae that ectoparasites, little is known about their life histories. hatch without well-developed eyes and tentacles may be Herein, we describe the adult morphology and develop- characteristic of the common ancestor of pyramidellids ment of the pyramidellid Odostomia columbiana, which and opisthobranchs; however, late development of the parasitizes the scallops Chlamys hastata and C. rubida larval heart is probably a derived condition of opistho- in the Northeast Pacific. Anatomically, adult O. colum- branchs. biana resemble other known pyramidellids although they lack the tentacular pads typical of other Odostomia Introduction species. Embryonic development is similar to that de- Pyramidellids are common ectoparasites in many ma- scribed for other pyramidellids: cleavage is unequal, gas- rine communities, but little is known about their biology trulation is partially by invagination, and considerable and life histories (Haszprunar, 1988; Wise, 1996). In par- growth occurs before hatching. However, embryonic and ticular, data concerning their development and larval bi- larval development are much slower than for other de- ology are lacking. As in many benthic marine inverte- scribed species. The planktotrophic larvae hatch after brates with limited adult dispersal, the duration of a 19 days of intracapsular development and metamor- planktonic larval stage may be a key factor influencing phose about 2 months later. O. columbiana veligers have pyramidellid distribution and population dynamics. a large black pigmented mantle organ to the right of the Long-lived planktonic stages may increase colonization midline, a distinct metapodial tentacle, and three or four of patchily distributed hosts, whereas species that hatch long bristles that project over the operculum from be- as crawling juveniles may have lower probabilities of lo- hind the foot. Observations of newly metamorphosed ju- cal extinction (Thorson, 1950; White et al, 1984; Gum- veniles suggest that previous disagreements regarding the ming, 1993). Therefore, knowledge of pyramidellid de- development of heterostrophy are due to variation in the velopment and larval biology is key to understanding degree of heterostrophy among species. Our observa- their life histories and host-parasite interactions, and to tions also generally corroborate certain scenarios ex- planning effective pest-control strategies for aquaculture. Pyramidellid development is also particularly infor- Received 16 August 1996; accepted 17 December 1996. mative when viewed in a phylogenetic context. Gener- * Address for correspondence: Committee on Evolutionary Biology, ally, pyramidellids are placed, with other families in the Culver Hall, 1025 E. 57th Street, Chicago, IL 60637; e-mail: rcollin®- order Heterostropha, at the base of the heterobranch midway.uchicago.edu Abbreviations: PMO = pigmented mantle organ; bp 1 = anterior buc- clade (Haszprunar, 1988). Although the relationships cal pump; bp2 = posterior buccal pump; VCSGL = ventral ciliated among these families are poorly resolved, pyramidellids strip gland. clearly represent basal members of the clade that in- 243 244 R. COLLIN AND J. B. WISE eludes opisthobranchs and pulmonates, and which is the which two or more developmental modes occur in one sister group of the Caenogastropoda (Haszprunar, 1988; species, may be an example of such taxonomic confu- Bieler, 1992; Mikkelsen, 1996). Knowledge of such basal sion. Boonea impressa has been reported to have plank- groups can be useful in tracing evolutionary transitions, totrophic development in North Carolina and lecitho- determining ancestral character states, and testing evolu- trophic development in Texas. However, these popula- tionary scenarios. This knowledge may be particularly tions may not represent the same species (Bouchet, useful in gastropod development, where evidence for ev- 1989). Both small eggs that develop into planktotrophic olutionary scenarios is often available from derived rep- larvae and large eggs with direct development have also resentatives of only a few clades. For example, van den been reported for Brachystomia rissoides in areas of Biggelaar (1996) found a trend in cleavage type and D- different salinities in Europe (Pelseneer, 1914; Rasmus- quadrant specification from equal cleavage and late spec- sen, 1944, 1951; Thorson, 1946). ification in "archaeogastropods" to unequal cleavage This paper adds to the data on pyramidellid develop- and early specification in opisthobranchs and pulmo- ment while avoiding the taxonomic confusion of previ- nates. Additionally, Page (1994) suggested that hetero- ous studies by describing both the adult morphology and chronic shifts in gastropod development decelerated the the development of the pyramidellid Odostomia colum- formation of larval and adult structures in opistho- biana Dall and Bartsch, 1907. This new information is branch larvae relative to prosobranchs. These scenarios discussed in the context of the general characteristics of were generated primarily from observations of derived pyramidellid development and its implications for the caenogastropods, opisthobranchs, and pulmonates. evolution of development within the gastropods. Moreover, detailed descriptions of pyramidellid devel- opment could be used to further support or refute these Materials and Methods types of scenarios. To date, accounts of pyramidellid reproduction and Mature Odostomia columbiana were found on the embryology are brief and limited to a small number of scallops Chlamys hastata and C. rubida dredged European (Lebour, 1932, 1936; Rasmussen, 1944,1951; from 90-130 m in San Juan Channel, Washington Fretter and Graham, 1949), eastern North American (48°34'10" N, 123°2'0" W) during March 1995. Both the (Robertson, 1978; White et al., 1985), and Indo-Pacific snails and their hosts were kept at ambient sea tempera- (Amio, 1963;Nishinocfa/., 1983; Gumming, 1993) spe- ture (8°-12°C) in flow-through sea-tables at Friday Har- cies (Table I). Although more than 50 species have been bor Laboratories, Washington. Snails ranged from 5 to reported from western North America (Dall and Bartsch, 8 mm in length and were identified as O. columbiana on 1909), there exists only a single account of pyramidellid the basis of the original description of the shell provided development from the region (LaFollette, 1979). No de- by Dall and Bartsch (1907) and by comparison with the tailed embryological studies of pyramidellids have been holotype and five syntypes on deposit at the National published, and larvae that do not metamorphose imme- Museum of Natural History (lot #126658). Voucher diately after hatching are seldom reared through settle- specimens have been deposited at the Field Museum of ment (see Robertson, 1967, for an exception). Conse- Natural History (FMNH 293334: dry shells; FMNH quently, the details of early development, the duration of 293334: formalin-fixed animals). the planktonic period, and the events at metamorphosis have been the subject of much speculation (Thorson, Adult morphology 1946; Fretter tf al, 1982). Adult snails were extracted for dissection by first Published accounts of pyramidellid reproduction and cracking their shells with a vise. Snails were dissected development are often obscured by taxonomic confu- whole, and structures of the gut and reproductive tract sion within the group. Criteria used to identify species were routinely stained with toluidine blue (Wise, 1993, are often not explicitly stated in published reports, or the 1996). taxonomy is so conjectural that species cannot be identi- fied with much certainty. For example, Clark (1971) and Development Hoffmann (1979) did not state how they identified their snails as Odostomia columbiana, and Cumming (1993) Egg masses were present on the scallops when they was unable to definitely identify "Turbonilla sp." to ge- were collected, and adult snails continued to produce egg nus or species. This taxonomic uncertainty makes it masses in the laboratory until May 1996. Scallop shells difficult to interpret comparative reproductive and de- were checked for new egg masses several times a month velopmental data. to determine whether reproduction was seasonal. Indi- Reports of poecilogony, a rare condition in gastropods vidual egg masses were removed from the scallop shells (Hoagland and Robertson, 1988; Bouchet, 1989) in and kept in small glass dishes while their development TRENDS IN GASTROPOD DEVELOPMENT 245 was observed. Although we did not observe egg laying, several egg masses were collected before the beginning of first cleavage. Observations of these egg masses were used to produce a developmental timetable. Eggs and egg cap- sules were measured prior to first cleavage, and measure- ments were taken in the plane of the chalazae. After hatching, larvae were transferred to filtered (0.45 /xm) seawater in small glass dishes. They were fed Isochrysis galbana and Rhodomonas sp. ad

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